Apparatus and method for producing curved electrostatic printing screens



CURVED July 8, 1969 J. w'. EDWARDS APPARATUS AND METHOD FOR PRODUCINGELECTROSTATIC PRINTING SCREENS Filed July 19, 1965 INVENTOR JAMES W.EDWARDS ATTORNEY APPARATUS AND METHOD FOR PRODUCING CURVED ELECTROSTATICPRINTING SCREENSv Sheet of 5 Filed July 19, 1965 A f I INVENTOR 9 JAMESw. EDWARDS ATTORNEY U.S. Cl. 9636.4 2 Claims ABSTRACT OF THE DISCLOSUREA method for making stencil screens used in electrostatic stencilprinting systems where the screens have masked and open areas enablingelectroscopic ink to pass through the opened areas of the screen to forma desired image pattern on a substrate. The method of making the screensincludes the steps of applying a photosensitive emulsion to a metalfabric, exposing the emulsion through an image pattern by means of lightand washing the nonexposed portion of the sensitive emulsion.Thereafter, the screen is bent to produce a desired cross-sectionalshape. The interstices of the non-image areas of the screen are filledwith a metal powder which is then pressurized to cause the powder tobecome impacted in the fabric interstices. The fabric is thereafterheated to cause the metal to become sintered to create a substantiallyrigid member. A modification is disclosed in which a non-metal fabric isfirst metalized and then treated as above. In addition, a pressurizingapparatus in the form of a plug-like element accommodates an articleinserted thereon. Pressure is applied from the interior of the pluglikeelement and the exterior atmosphere surrounding the plug-like element sothat pressure can be applied op posite the surfaces of the article.

This invention relates in general to certain new and useful improvementsin the method of making electrostatic printing equipment, and moreparticularly to an improved method and apparatus for makingelectrostatic printing screens having curvilinear shapes and which areused in electrostatic screen process printing.

The presently known techniques in electrostatic printing are describedin U.S. Letters Patent No. 3,081,698 which relates to a method ofelectrostatic printing by elimination of pressure or contact between theprinting element and the subject material being printed. This techniqueinvolves the transfer of a resinous based ink through an electrostaticfield to an image-receiving media. The ink or pigments are usually inthe form of a fine powder having a particle size which is small enoughto pass through the interstices of the open areas of a stencil orso-called screen. A roller or similar mechanical device normally carriesthe ink particles to a point in close proximity to the stencil and wherethe ink is carried through the stencil by the electrostatic field to theimage-receiving media. When the voltage is applied to the roller orelement carrying the pigment, the particles acquire a charge. The chargeis, of course, opposite to the backing plate and the ink particles are,therefore, accelerated through the openings or interstices in the openareas of the screen and toward the image-receiving media. Theimage-receiving media may consist of a mandrel which serves as acounter-electrode and which is capable of retaining the article to beprinted. Thereafter, the ink will collide with and adhere to the articlewhich is to be printed and the image is subsequently fixed by heat orsolvent or a vapor or by other suitable means which are known in theprior art.

Since the initial development of the theory of electro- United StatesPatent 'ice static printing, there have been many attempts to print byelectrostatic process principles on items having curvilinear shapes.Most of these attempts proved to be unsuccessful, for a number ofreasons. In all of the electrostatic screen process printing apparatusthus far developed, the apparatus has necessitated the transfer of inkacross a definite and appreciable space. However with curved articles,all portions of the screen were not equidistantly spaced from thesurface of the article to be printed. As a result thereof, the printingwas weak in some areas and extra heavy in other areas. Furthermore,because of this variable spacing between the screen and the substrate tobe printed, the problems of image distortion effects arose.

It has recently been discovered that it is possible to printcurvilinearly shaped articles using electrostatic screen processprinting techniques by the employment of a curved electrostatic printingscreen. It was recognized that it is necessary to employ a screen havingsubstantially the same size, shape and contour of the article beingprinted, in the areas where the article is to receive the print.Moreover, it has been found that very good results have been obtainedwhen the article to be printed is completely surrounded by anelectrostatic printing screen and where all portions of the inkreceiving surface of the article are equidistantly spaced from allpoints on the screen. However, to date, there has been no known andeffective method of producing an electrostatic printing screen having acurvilinear shape which can be employed in electrostatic screen processprinting techniques. Furthermore, there is no known method of producinga self-supporting screen which is capable of surrounding the article tobe printed and yet being spaced sufiiciently to maintain anelectrostatic field therebetween. Accordingly, it was necessary toproduce screens with external tensioning devices which were massive andawkward to handle. Furthermore, it was difficult to adapt ink feedingmechanisms to these curved screens external members. The printingscreens of this type were quite costly to manufacture and moreover, werenot constructed with the desired degree of tolerance usually required inelectrostatic screen process printing techniques.

It is, therefore, the primary object of the present invention to providea method of making electrostatic printing screens for use inelectrostatic screen process printing techniques.

It is another object of the present invention to provide a method of thetype stated for producing electrostatic screens having self-supportingcurvilinear shapes.

It is a further object of the present invention to provide a method forproducing electrostatic printing screens of the type stated whichrequires a minimum amount of manual attention and thereby permitsconstruction of a low cost electrostatic printing screen.

It is an additional object of the present invention to provide a methodfor producing screens of the type stated where the screens arecharacterized by simplicity, dependability, ruggedness and low cost.

It is also an object of the present invention to provide an apparatusfor making curved electrostatic printing screens to be used inelectrostatic screen process printing techniques.

It is another salient object of the present invention to provide anapparatus of the type stated which is relatively economical tomanufacture and requires a minimum of manual attention in its operation.

With the above and other objects in view, my invention resides in thenovel features of form, construction, arrangement and combination ofparts presently described and pointed out.

In the accompanying drawings (3 sheets):

FIGURE 1 is a perspective view of a wire mesh screen coated with aphotosensitive emulsion and with a desired image projected thereon by asource of light through a negative having the desired image pattern;

FIGURE 2 is a perspective view of a hardening bath showing the wire meshscreen of FIGURE 1 submerged therein;

FIGURE 3 is a perspective view of the wire mesh screen formed into atruncated conically shaped section which is substantially similar insurface contour to the articles to be printed;

FIGURE 4 is a vertical sectional view, partially broken away, of a plugor die frame which is constructed in accordance with the presentinvention and supports the conically shaped wire mesh screen in aliquid-metal bath;

FIGURE 5 is a vertical sectional view, partially broken away, showing aunique type of cooperating die frame constructed in accordance with andembodying the present invention for producing the conically shapedelectrostatic printing screen of the present invention;

FIGURE 6 is a fragmentary sectional view taken along line 66 of FIGURE5;

FIGURE 7 is a side elevational view, partially broken away, showing theconically shaped electrostatic printing screen sintered in a furnace;

FIGURE 8 is an exploded perspective view showing the method of attachingend supporting rings to the conically shaped electrostatic printingscreen;

FIGURE 9 is a perspective view of the final electro static printingscreen; and

FIGURE 10 is a fragmentary sectional view taken along line 10- 10 ofFIGURE 9.

Generally speaking, the present invention provides a method andapparatus for producing curved screens to be used in electrostaticprinting operations. Generally, the curved screens are of the typedescribed in copending application Ser. No. 472,982, filed July 19,1965, now US. Patent No. 3,302,561, which relates to electrostaticscreen process printing on curvilinearly-shaped articles. In the processof the present invention a direct screen photosensitive emulsion isapplied to a wire mesh which is preferably held in a screen chase. Thescreen is then exposed to light through a photographic negative of therequired print or design which is ultimately to be produced on asubstrate. The wire mesh is thereafter washed in hardening bath which isdesigned to harden the exposed portion of the photosensitive emulsionand rinse away the non-exposed portion of the emulsion, thereby leavinga positive image on the screen. In other words, the image areas will befilled with the exposed photosensitive emulsion since this area wasexposed to the light. Thereafter, the metal may be cut as desired andformed into a desired shape, such as a truncated-conically-shapedsection, which will surround and generally conform to the overall shapeof a container which is ultimately to be printed. The end margins of thesection may be tacked or spot welded in order to form the truncatedsection. Thereafter, the comically-shaped screen is supported on asuitable pluglike mandrel-shaped die which is especially designed forthe purpose of the present invention. The screen and mandrel-die is thendipped into a suspension or container of finely powdered metal where adesired metal or alloy is filled in the interstices of the open meshportions of the screen. Thereafter, the mandrel-die supporting thecomically-shaped screen is withdrawn from the metal suspension andpermitted to sufficiently dry, where the metal particles remain in theinterstices of the open mesh portions of the screen.

The conically-shaped screen is then fitted between the mandrel-die whichhas an outer rubber surface and a second die consisting of a fairly hardrubber sleeve which surrounds the screen. The mandrel-die, sleeve andscreen are all secured by a band at their upper ends thereby providing apneumatic seal between the first and second comically-shaped rubbersections which retain the screen. The mandrel-die is provided withsuitable fluid apertures permitting communication from the mandrel tothe inner rubber mold. The apparatus is then submerged in a fluidpressure tank and the pressure is increased so that the metal in theinterstices of the screen is impacted by the pressure on both surfacesof the comically-shaped rubber dies.

After the metal has been sufficiently impacted in the surfaces of thescreen, the screen is then placed in a suitable furnace for heating thescreen at least to the annealing temperature of the metal impacted inthe interstices so that the metal is sintered into the screen. After themetal has been hardened, the screen has a fairly rigid shape and may besubjected to a final shaping, if desired. End rings are then placed onthe opposite ends of the screen to provide further rigidity.

Referring now in more detail and by reference characters to the drawingswhich illustrate a preferred embodiment of the present invention, theprocess of the invention and the electrostatic screen produced therebyare more fully illustrated in detail. In FIGURE 1, a metal fabric orso-called wire mesh 1 is illustrated. The metal fabric may be of theplain weave or the twill weave. However, in either weaves the openingsare square. For the purposes of electrostatic screen process printing,the metal fabric should be formed of a No. No. 350 wire mesh. In unusualcases, coarser cloths than No. 80 may be employed and similarly inunusual cases, finer cloths than No. 350 may be employed. The wire meshnumber is the standard employed in the metal fabric industry Where forexample No. 80 square mesh cloth would have 80x80 openings per squareinch or 6,400 openings per square inch while No. 300 metal cloth wouldhave 90,000 openings per square inch. The wire mesh 1 may be formed ofany suitable metal which has the desired gauge size and which iselectrically conductive, at least for purposes of electrostaticprinting. An electrostatic field is to be maintained between a screenand a counter-electrode. Such suitable metals to be used are nickel,aluminum, stainless steel, copper, etc. Similarly, suitable alloys ofmetals may be employed such as a copper-zinc allow, or a copper-aluminumalloy, etc.

Thereafter, a suitable direct screen emulsion or sensitized photographiccoating is applied directly to the surface of the metal fabric 1.Sensitized photographic coatings such as polyvinyl alcohol, polyvinylacetate, modified polyvinyl plastics, commercial coatings, gelatinouscoatings, transfer-type sensitized films and knife-cut fil ms may bedeposited on the metal screen fabrics. The important step before theapplication of the sensitized coating is to insure that the metal fabricis cleaned perfectly. Since the metal is non-absorbent, it is essentialthat the fabric be clean and devoid of any oils or greases in order thatthe emulsion film or coating used is well adhered to the surface of thewire mesh 1. Generally, the metal fabric 1 may be cleaned by washing thesame in a 5% to 10% glacial acetic acid solution for approximately 5 to10 minutes and then rinsed well with hot water. The formulas forsensitizers may vary considerably and the most desirable type ofsensitizer must be selected. One of the most eificient sensitizers foundfor use in the present invention contains ammonium bichromate, potassiumbichromate, ammonium hydroxide, glycerin solution, and distilled water.

The metal fabric is then exposed to a proper light source through aphotographic negative of the required print or design, such as shown inFIGURE 1. It can thus be seen by reference to FIGURE 1 that the metalfabric 1 is exposed to a proper white-light source 2 through a negative3 having the proper design or image pattern which is to be formed on themetal fabric 1 such as illustrated by reference numeral 4. Generally,the metal fabric 1 may be suitably mounted within a desired frame orchase for ease of handling. The sensitized metal fabric 1 may then beexposed in either a dry or damp state depending on the type of screenthat is being prepared. The length of the exposure is determined byexperimentation but may vary anywhere from about 5 to 25 minutesdepending on different factors. The type of light employed for exposingis very important and must be uniform in actinic intensity. In otherwords, the light energy should produce uniform chemical change in allareas during all times that the light is on. Generally, conventionalphotoflood lamps and are lamps provide suitable results.

After the sensitized metal fabric 1 has been exposed to the light source2, it is then developed in a hardening or so-called developing tank 5substantially as shown in FIGURE 2. Development generally takes place inhot Water with a temperature within the range of 95 to 115 F. The waterpreferentially washes away or dissolves all of the unexposed portions ofthe photosensitive emulsions. The portion of the emulsion residing inthe image 4, which has been exposed to light becomes hardened and is nowimpervious to the wash treatment and remains on the metal fabric 1. Thetank 5 is preferably made of glass, Bakelite, stainless steel, or a tankwhich is porcelain finished on the interior surface. The tank may berocked gently forcing the rocking of the water to flow over the metalfabric 1 and provide some slight agitation.

After the developing operation in the tank 5, it can be seen that thesensitized metal fabric 1 has a positive image 4 on the surface thereof.It is often desirable for electrostatic printing techniques to build upa sufiiciently thick coating of an emulsion on the image area so that asufficiently large relief may be obtained between the non-printing andthe printing areas on the final electrostatic printing screen.Accordingly, the above operation for producing the positive image 4 onthe surface of the screen may be repeated. In this process, aphotosensitive emulsion is again applied to the surface of the screen byany of the suitable techniques aforementioned. Thereafter, thesensitized fabric 1 is exposed to the same light source or imagepattern. In this case, however, the fabric 1 will have to be registeredin position so that the image projected through the negative on therepeat operations is marginally registered with the positive image orpattern 4 on the surface of the fabric 1. After the sensitized fabric 1is again exposed, it is hardened in the developing tank 5 so that theemulsion within the positive image pattern 4 is hardened and theremaining emulsion is washed away.

It has been found in connection with the present invention that anon-metallic fabric may be employed if the fabric is later gmetallizedin some fashion. In the event that a non-metallic fabric is employed,the fabric is preferably removably mounted in a frame until the processof forming the fabric into a sufficiently rigid metal structure has beencompleted. The fabrics which may be employed in the construction of theelectrostatic printing screens are silk, organdy, cotton, nylon, Dacron,vinyl, Vinyon, linen, etc. The fiber of the fabric may be made ofanimal, plant, mineral synthetic material or combinations of the abovematerials. However, of all of the fabrics attempted for use, a syntheticcloth containing monofilament strands such as cloth sold under thetrademark nylon has been found to be most suitable. The cloth fabricsemployed may be gauze weave, a leno weave, or a.plain weave. The gauzeweave and leno weave are preferred since both produce relatively strongcloths. This type of fabric is then preferably metalized in a suitablemetalizing chamber such as by vacuum metal vapor deposition processtechniques. Thereafter, the process of producing electrostatic screensof the present invention is the same as one originally starting with ametal fabric.

The fabric 1 is next formed into a desired shape. The metal fabric 1,which is fairly thin still has sufiicient rigidity to be formed by hand,and to maintain its shape. For purposes of illustrating the presentinvention, the metal screen has been shown to be fonmed into a truncatedconically shaped element substantially as shown in FIGURE 3. As the endsof the fabric 1 are turned, they may be tack welded or soldered in themanner as shown 6 in FIGURE 3. In order to obtain the desired shape, thefabric 1 may be formed about a mandrel or container having a size orshape similar to the desired size and shape of an article which isultimately to be printed. For example, if it was desired toelectrostatically print on the side walls of disposable thin-walledcontainers, such a container or the mandrel therefor could be used asthe die upon which the fabric 1 is formed into the desired shape.

The \metal fabric 1 can then be made into a rigid structure by suitablepowder metallurgy techniques. The fabric 1 in the form of the truncatedconically shaped element is then suitably mounted on a plug or innermold 6 to be hereinafter described in more detail.

The metal fabric 1 which is mounted on the plug or soealled mandrel typemold 6, is then suitably introduced into a powder metal solution in theform of a slurry 7 contained within a suitable tank 8 in the manner asshown in FIGURE 4. The powder metal is preferably suspended in adesirable carrier, such as water or any similar inert material whichwill carry the metal powder and is capable of being removed therefrom bydrying. The powders which are capable of use in the present inventionare powders of iron, nickel, copper, etc. and almost any metal powderwhich is capable of being hardened by powder metallurgical techniques.It is also possible to use composite materials, such as metal-metalcombinations as the metal powder. Such materials which may be used aretungsten-silver, tungsten-copper, molybdenum-silver, andmolybdenumcopper, etc, The compositions may vary within wide limitsaccording to the properties required in the finally finished product.For example, commercial tungstate-silver compositions may contain from10 to 70% silver. Also, compositions containing tungsten and 10% silverare also commercially available and have a density of about 17.5 gramsper cubic centimeter. It is also possible to employ metal compositionsconsisting of three metals such as tungsten-nickel-copper. For example,a commercially available tungsten-nickel-copper composition existing in90% tungsten, 7.5% nickel and 2.5% copper is available. By varying thecompositions, it is possible to produce a material having a desireddensity. It is also possible to employ metal-non-metal combinations,such as coppergraphite and bronze-graphite material. These compositesare produced by mixing fine copper powders or bronze powders with 5 to70% graphite powders. Also, copperzinc and copper-tin brasses are verysuitable. In some cases, a suitable binder such as tar may be added tosecure satisfactory coherence of the product. Thus, it can be seen thata wide range of metals and non-metals combinations are available for usein the powder metallurgy techniques.

It should be understood that the metal powder may be applied to thesurface of the fabric 1 by any suitable technique, such as dipping orcoating. Moreover, the metal may be applied by wet brushing or by use ofa spatula, if desired. Moreover, the metal may be in the form of a thickslurry, which may be painted on. However, in any event, care should betaken to remove any excess material from the image area 4.

As thefabric 1 is introduced into the metal slurry 7, the metalparticles will fill in the interstices of the nonprinting areas, butwill not adhere to the area in the printing image 4 inasmuch as thelatter has the photosensitive emulsion thereon. A momentary introductioninto the metal suspension 7 is all that is necessary in order tocompletely fill the interstices of the non-printing areas. After the die6 has been removed from the metal suspension 7, the fabric 1 ispermitted to dry so that the Water carrier may be completely removedfrom the metal. It has been found that the screen may be dried as itstands right on the mandrel-mold 6 or may be removed therefrom asdesired. Drying will take place at a faster rate if the screen isremoved from the mold 6. It has been found that air drying forapproximately 30 minutes at room temperature conditions is sufficient.However, the fabric 1 may be dried in an oven at approximately F. forapproximately 10 to 15 minutes for complete drying.

The dried metal fabric 1 is then suitably mounted on a metal powdercompacting apparatus 9 substantially as shown in FIGURE 5, the mold 6forming a part thereof. The apparatus 9 is hereinafter described in moredetail. However, it can be seen that the compacting apparatus 9 issuitably introduced into a high pressure fluid tank 10 and carries themetal fabric 1. The high pressure tank 10 may be of any conventionalconstruction and is, therefore, not described in detail herein. Thepurpose of the compaction process is for the shaping of the powder inthe metal fabric 1 to coherent bodies which have sufficient strength topermit handling and safe transfer to a sintering furnace. Moreover, thecompaction process permits the metal powders impacted in the intersticesof the fabric 1 to have a uniform density and other characteristicswhich are essential for satisfactory sintering. Furthermore, the highpressure techniques permit the metal powders to become permanentlyembedded in the interstices of the metal fabric 1. It has been found inconnection with the present invention that desirable results areobtained if the metal fabric 1 is subjected to pressures within therange of to 100 tons per square inch until the metal powder is wellcompacted. The pressure required, however, is somewhat variabledepending upon the particular metal powder employed and the metal fabric1 which is employed.

After the metal powder has been suitably compacted in the interstices ofthe metal fabric 1, the fabric 1 in the form of its truncated conicalshape is suitably placed in a sintering furnace 11, substantially asshown in FIGURE 7. The furnace 11 is conventional in its constructionand is, therefore, not described in detail herein. However, it ispointed out that the furnace is constructed of a desirable ceramicmaterial which is capable of withstanding the temperatures necessary forsintering of the metal powder contained in the metal fabric 1.

The theory of sintering is well known and is, therefore, not set forthin detail herein. However, it is pointed out that by sintering, themetal powders can be transformed without fusion to a compact materialthat exhibits a strength of the same order of magnitude as cast andwrought materials of corresponding composition. The powder compacts aresintered by heating to temperatures of about two-thirds of the absolutemelting temperature and at least to the annealing temperature of themetal powder employed. With some metals, considerably highertemperatures are employed. For example, the refractory metals such astantalum, molybdenum, and tungsten are sintered at a temperature veryclose to their melting points. In homogeneous systems, as well as in themixtures of metals with non-metals, sintering is preferably formed inthe absence of a liquid phase. In mixtures of different metals,sintering may be performed either in the absence or in the presence of aliquid phase.

It is well understood that satisfactory sintering depends upon closecontrol of the sintering atmosphere as well as of the heating cycle,that is the rate of heating, maximum temperature, time of maximumtemperature, and rate of cooling. However, there are a number ofcommercially available apparatus necessary for performing this functionand they are neither illustrated nor described in detail herein. In thecase of the present invention, it is preferred that sintering isperformed either in a neutral atmosphere or a reducing atmosphere or ina vacuum. Such suitable neutral atmospheres may be in the presence ofnitrogen, argon or helium. Suitable reducing atmospheres are carbonmonoxide, hydrogen, disassociated ammonia, natural gas, coke-oven gasand partially 'burnt hydrocarbons. The metal fabric 1 remains in thesintering furnace 11 for a time which is determined as previouslymentioned by the type of metal powder employed and the type of metalfabric 1 employed. After the sintering process has taken place, thefurnace 11 is allowed to cool for ultimate removal of the metalfabric 1. The fabric 1, which has been sintered, is shown in crosssection in FIGURE 10. -It can be seen that the powdered metal iscompletely dispersed throughout the interstices of the screen and formsa substantially rigid structure. The fabric 1 is thus formed into asuitable electrostatic printing screen 12. The screen 12 may bepreferably provided with metal end rings 13 as shown in FIGURE 8 toproduce the final screen 12 as shown in FIGURE 9. The metal end ringsmay be attached by soldering or tack Welding to the upper and lowermargins of the conically-shaped fabric in the manner as shown in FIGURES8 and 9.

It should be recognized that the excess photo-sensitive emulsion whichoriginally resides in the image pattern 4 is removed during thesintering operation. Because of the high temperatures, thephotosensitive emulsion is disintegrated, thereby leaving an open areain the image pattern 4. Furthermore, it should be understood that it isalso possible to remove the photosensitive emulsion in the image pattern4 by a differential etching solution. Any of a number of commerciallyavailable emulsion removers may be employed. It should also berecognized that the metal fabric 1 may be formed into double curvatureand more complex shapes as desired by such operations as drawing,punching and molding at various stages of the preparation.

It is also possible to slightly modify the process of the presentinvention by eliminating the powder metallurgical technique andsubstituting therefor, the use of polymerizable plastics or variouspolymers, such as polycaprolactam. Other materials which may be employedare epoxy resins, plaster of Paris, porcelain cement, etc. In fact, anyof a number of materials which are capable of polymerizing and hardeningin the interstices of the metal fabric 1 can be suitably employed.

Some of the polymers which may be useful in the present invention arethe polyalkenes formed of such monomers as ethylene, propylene,isobutylene, polydialkenes formed from monomers such as butadiene andisoprene, the halogenated polyalkenes formed from monomers such astetrafluoroethylene, vinyl resins such as polyvinylacetal,polyvinylacetate, polyvinylchloride, polystyrenes formed from suchmonomers as styrene, etc. Another class of synthetic polymers capable ofbeing used in the present invention are those of the cellulosederivatives, e.g. cellulose esters such as cellulose acetate, cellulosetriacetate, cellulose propionate and acetal resins such as those formedby addition polymerization of formaldehyde or higher aldehydes. Alsouseful are the phenolic resins formed by the condensation of phenol,cresol, xylenol, and other substituted phenols with formaldehyde orhigher aldehydes. Other polymer systems which can be used are thoseobtained by the polymerization of furfural or furfuryl alcohol and bycopolymerization of furfural and a ketone. Various other polymer groupsare also useful in the present invention such as the polyurethane resinsprepared by the reaction of polyols, polyesters, etc. and polymers ofthe polyamide type produced by condensation of diamines withdicarboxylic acids. The polymers may be linear or crosslinked.

It has been found that screens of this type find particular utility forelectrostatic printing in the type of apparatus described in mycopending application Ser. No. 472,982, filed July 19, 1965. In theapparatus and method described in said copending application, anelectrostatic printing screen is disposed circumferentially around thearticle to be printed, which is generally a disposable plasticthin-walled container. Generally, the substrate has a curvilinear shape,such as a comically-shaped container and the counter-electrode in theelectrostatic printing apparatus is generally in the form of a mandrelfor sup porting the container and which has the same size and shape asthe container. Thus, the container is mounted on the mandrel in spacedrelation to the screen. In the case of electrostatic printing with thescreen of the present invention, the screen is circumferentiallydisposed about the container and in close proximate relationshipthereto,

where ink can be applied to the open mesh portions of the screen 12 anddeposited on the surface of the container. It has also been found thatelectrostatic printing can be performed on curvilinear surfaces wherethe screen has a surface contour in at least one plane which issubstantially identical to at least one portion of the surface contourof the element being printed, such as in the method described in mycopending application Ser. No. 472,982, filed July 19, 1965. In thislatter process, the screen is rotated or oscillated in timed relation tothe rotation or oscillation of the element being printed.

In this manner, the container surface tangentially approaches andtangentially departs from the screen so that printing will occur throughthe screen along an elemental line of closest approach. The container isgenerally rotated at approximately the same rate of speed as therotation or oscillation of the screen through this line of closestapproach so that a continuing line of tangency occurs between thesurface of the container and the surface of the screen. A desiredelectroscopic ink is fed through the screen along this line of closestapproach by any suitable ink feeding mechanism. Furthermore, by use ofscreens produced by the method of the present invention, it is possibleto employ contact electrostatic printing on curvilinear surfaces. Thismethod is again described in the aforementioned copending application.

Any of a variety of electroscopic inks can be used Where the screenproduced by the present invention is used in the electrostatic printingoperations. Generally, the electroscopic inks comprise a finelydispersed powder which is capable of being triboelectrically charged.The powder generally carries a desired pigment. A number of satisfactorypowders can generally be employed in the electrostatic printingoperations and each must be in a finely divided state. Suitable powdersare dyed thermoadhesive resins such as rosin, gum copal, gum sandarac,ethyl cellulose, Egyptian asphalt and the like. A very satisfactorythermoadhesive powder can be produced by dissolving equal parts of ethylcellulose and vinsol resin in acetone together with a small amount ofspirit soluble aniline dye such as'Nigrosine or aniline blue and spraydrying the solution to produce an extremely fine powder havingsubstantially spherical particles. Dyed Lycopodium powder is suitablewhere thermoadhesive properties are not required of the powder, as isalso starch, cellulose flour, powdered metal and copper powder.

Whether fusible, thermoadhesive or non-fusible powders or others areused, the particle size is preferably near the limit of definition ofthe eye under ordinary reading conditions. Excessive powder sizecontributes to graininess in appearance of the image. n the other hand,extremely fine powder may be undesirable in many instances due to itstendency to ball up or cling together in clusters. It is, therefore,desirable to use a powder in which substantially all the particles arewithin the size range of 2 to microns. If spherical powders are used,this refers to their diameters, otherwise to the largest dimension. Formost purposes, it is preferred to use an equidimensional powderparticle, the sphere being the preferred form.

As previously indicated, when the screen produced by the presentinvention is employed in an electrostatic printing operation, voltage isapplied to the counterelectrode which holds the substrate, the screenand to a feeding electrode. The charge in the counter-electrode isopposite to the charge on the screen so the particles are propelledthrough the screen openings or interstices of the screen toward thecounter-electrode. The image is then generally subsequently fixed byheat, a solvent or a vapor, or by any other suitable means dependingupon the type of pigment powder which has been employed and the natureof the material being printed.

The current source employed in electrostatic printing operations isadapted to develop a relatively high direct current potential. While thecurrent requirements for electroprinting of the type here employed arenot heavy in the ordinary sense, a very definite electron current orspace current flows across the printing space during the printingoperation. It is desirable to have a space current of at least 1 to 2milliamperes per square inch of printing area. Moreover, the highpotential source should be capable of maintaining a desired voltageunder current range in the range of approximately milliamperes orslightly more.

It should be recognized that the electrostatic screens produced by thepresent invention have the capabilities of withstanding this type ofhigh voltage potential applied thereto. Moreover, they are capable ofwithstanding abrasion of the rough handling by virtue of theirconstruction.

The compaction apparatus 9 illustrated in FIGURES 5 and 6 may beconsidered as a molding press and generally comprises the mandrel typedie or plug 6 which generally has the surface contour similar to thesurface contour of the screen being produced. In the case of the presentinvention, for example, the plug 6 has a surface contour, shape and sizewhich is substantially similar to the mandrel upon which the containerto be printed would be mounted and substantially similar to the screen'12. Suitably disposed around the outer surface of the plug 6 is asomewhat flexible but sufficiently rigid pressure forming die 14. Thedie 14 is secured at its upper and lower ends to the exterior surface ofthe plug 6 by means of any suitable adhesive, such as an epoxy resin 15,thereby forming a suitable air space 16 between the pressure forming die14 and the plug 6. The plug 6 is provided with a hollow fluid chamber17, which has a series of fluid apertures 18 in its side wall providingcommunication between the air space 16 and the fluid chamber 17.Morever, the plug 6 is provided on its upper surface, reference beingmade to FIGURE 5 with a series of fluid ports 19 providing communicationbetween the fluid chamber 17 and an exterior atmosphere such as theinterior of the pressure tank 10. The plug 6 is also provided with astem or handle 20 in the form of a tube which extends through the bottomwall of the plug 6 and communicates with the space between the exteriorsurface of the plug 6 and the pressure forming die 14. The stem 20 isalso provided with a conventional type of valve 21 which may be openedand closed as desired for removing air from the space 16.

The plug 6 is preferably constructed on any rigid metal such asstainless steel, aluminum and should be capable of withstanding the highpressures normally encountered in compaction operations in powdermetallurgy techniques. The pressure forming die 14 is preferably formedof any rigid rubber material which is capable of providing sufiicientflexibility when subjected to pressures within the range of 5 to 100tons per square inch. It has been found that suitable rubber materialswhich may be employed are neoprene rubber, polyvinyl butyral, Buna-Nrubber, sodium polybutadiene, and many naturally occuring rubbers. Manysynthetic rubber polymer combinations such as butadiene and styrene havealso proved suitable for forming the pressure forming die 14. Butyralrubber, produced by the polymerization of isobutylene and a small amountor isoprene at approximately F. in the presence of a Friedel-Craftscatalyst has also been found to be useful. Similarly, special rubbersconsisting of copolymers of butadiene and acrylonitrile have proved tobe very useful. Additionally, some polysulfide and silicon rubbers whichpresent sufficient rigidity and yet sufiicient flexibility for positivepressure forming have been found to be useful for the purposes of thepresent invention.

Disposed around the positive pressure forming die 14 is the metal fabric1 which is in the form of the screen 12 and disposed about the fabric 1is a female positive pressure forming die 22 which has the same surfacecontour and shape as the fabric 1 and the die 14. Thus, it can be seenthat the fabric 1 is held between the two pressure forming dies 14 and22. Moreover, the pressure forming die 22 is formed with the sameflexibility as the die 14 and of the same material and same thickness.It has been found in connection with the present invention that wheneach of the dies 14, 22 is formed with an overall thickness ofapproximately 0050-0125", very suitable results have been obtained. Eachof the elements, namely the plug 6 with the positive pressure formingdie 14 secured thereto, the fabric 1 and.the die 22 are all secured bymeans of a releasable clamp 23. The band 23 is preferably provided witha releasable tightening mechanism such as a screw type clamp, where eachof the aforementioned elements can be assembled in the manner as shownin FIGURE 5. In this manner, a pressuretight air seal is maintainedbetween the pressure forming dies 14 and 22.

.In' use, the fabric 1 is disposed about the pressure forming die 14 andthe outer pressure forming die 22 is disposed around the fabric 1 andeach of the elements is secured by means of the removable clamp 23.Thereafter, the area between the two dies 14, 22 is evacuated by openingthe valve 21 and connecting the stem 20 to a suitable vacuum. Evacuationof this area occurs through the stem 20. After a sufiicient quantity ofair has been removed from this chamber, the valve 21 is closed.Thereafter, the entire assembly is inserted into a pressure vessel, suchas the tank 10 which is closed and subjected to high hydrostaticpressure. It can be seen that the fabric 1 is maintained in afluid-tight chamber between the two pressure forming dies 14, 22. Thefluid under pressure will flow through the fluid ports 19 into the fluidchamber 17 and through the apertures 18 into the air space 16. As thisoccurs, the fluid under pressure will force the male pressure formingdie 14 into contact with the interior surface of the conically shapedfabric 1. Pressure against the outer surface of the female positivepressure forming die 22 will force the same into contact with theexterior surface of the fabric 1, thereby compacting the metal powdersinto a solid homogeneous metal fabric. After the compaction operationhas been completed, the entire assembly may be removed and the valve 21opened, permitting disassembly and removal of the metal fabric 1.

It should be understood that changes and modifications in the form,construction, arrangement and combination of parts presently describedand pointed out may be made 12 and substituted for those herein shownWithout departing from the nature and principle of my invention.

Having thus described my invention, what I desire t claim and secure byLetters Patent is: Y

1. The method of making electrostatic printing screens having opened andclosed areas for use in electrostatic printing processes, said methodcomprising metallizing a non-metallic fabric, applying an image patternto said fabric creating image and non-image areas, bending said fabricto produce a desired cross-sectional shape, filling the interstices ofthe non-image areas of the fabric with a metal containing powder,pressurizing the powder in the fabric to cause the powder to becomeimpacted in the fabric interstices, and heating the fabric therebycausing the metal to become sintered, thereby creating a substantiallyrigid electrostatic printing screen.

2. The method of making electrostatic printing screens having opened andclosed areas for use in electrostatic printing processes, said methodcomprising metallizing a non-metallic fabric, applying a photosensitiveemulsion to said fabric, exposing the emulsion with an image patterninterposed between the fabric and the means of exposing, removing thenon-exposed portions of the photosensitive emulsion thereby creatingimage and non-image areas, bending said fabric to produce a desiredcross-sectional shape, filling the interstices of the non-image areas ofthe fabric with a hardenable material, removing the remainingphotosensitive emulsion, and causing the material to harden therebycreating a substantially rigid electrostatic printing screen.

References Cited UNITED STATES PATENTS 2,213,237 9/1940 Brennan et al.l0l128.2 X 2,288,020 6/1942 Noland et al 101-1282 2,573,951 11/1951Brennan 101128.2 2,860,576 11/1958 Short 9636.4 X

FOREIGN PATENTS 756,315 9/1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

R. E. MARTIN, Assistant Examiner.

U.S. Cl. X.R.

